Oscillators using resonators, such as crystals, are typically used to provide precise, stable time references. Sometimes an integrated circuit chip, device or appliance, may require more than one oscillator, such as when multiple operating modes benefit from different clock frequencies. For example, in battery operated appliances, such as cell phones, it is advantageous to shut off a high frequency (13 MHz for example) crystal oscillator when the high frequency crystal oscillator is not needed, and maintain a low power 32 kHz oscillator running to keep time and permit the system come out of a sleep state.
FIG. 1 is a schematic of a conventional Pierce oscillator circuit used to implement a single crystal-based oscillator circuit on an integrated circuit chip. The oscillator circuit 100 on integrated circuit chip 102 includes resistor 104, inverter 105, and capacitor 106 integrated on the integrated circuit chip. Capacitors 108 and 110 can also be integrated on the integrated circuit chip, but do not have to be, as the location of the capacitors 108 and 110 makes no difference to circuit behavior. Crystal 112 is an external component that is soldered, or otherwise attached to the rest of the circuit. Crystal 112 is typically made of quartz, or some other piezoelectric material. The Pierce circuit architecture requires two bond pads, 114 and 116 in order to attach crystal 112 to the Pierce circuit. Such a Pierce circuit is very easy to design, test, and manufacture.
Resistor 104 is typically of a high value, usually a million Ohms or more, and is used to force inverter 105 to a high gain point. This lets inverter 105 behave as an amplification device, and not as a traditional inverter. Capacitor 106 is tied to the high impedance node of inverter 105 and prevents any leakage current in the circuit from disturbing the DC bias point of inverter 105, and hence the amplification of inverter 105. Capacitors 108 and 110, along with parasitic resistance in the circuit, provide phase shift around the circuit loop 113 to ensure oscillation of crystal 112 will grow.
FIG. 2 is a schematic diagram of dual Pierce oscillator circuits used to implement crystal-based oscillator circuits on an integrated circuit chip. As shown, a first Pierce oscillator circuit 200 and a second Pierce oscillator circuit 202 are implemented separately on a single integrated circuit chip 204. As the two circuits are implemented separately, each has its own set of pads, such as pads 206 and 208 for Pierce oscillator circuit 200 and pads 210 and 212 for Pierce oscillator circuit 202. This requires four pads to make all the necessary connections to crystals 214 and 216. In the case of FIG. 2, crystal 214 might be resonant at 32768 Hz (32 kHz), and crystal 216 might be resonant at 13 MHz.